1. Field of the Invention
The present invention relates to a semiconductor laser modulation control system and a method for driving and modulating a semiconductor laser in accordance with input data.
In known digital optical communication systems, a direct modulation system is employed by which a current driving a semiconductor laser is controlled in accordance with input data. Also, due to the development of low-loss single mode optical fibers, which enable a high-speed data transmission, the semiconductor laser must be modulated and driven stably and at a high speed.
2. Description of the Related Art
In a conventional semiconductor laser direct modulation system, a current nearly equal to a threshold current needed to start a laser emission is supplied to a semiconductor laser as a bias current, to place the laser in a low intensity state, and when an input data is "1", for example, a drive current is superposed on the bias current to place the laser in a high intensity state. Namely, the intensity modulation is effected in such a way that the high intensity state is realized when the input data is, for example, "1", and the low intensity state is realized when the input data is, for example, "0", and therefore, a two-level optical signal is transmitted through an optical fiber to a receiving side.
Note, the low intensity state includes a very low intensity state at which the light emission is negligible.
FIGS. 1A to 1C are diagrams explaining a conventional example in which the high intensity state is realized when the input date is "1", the low intensity state is realized when the input date is "0", and RZ (Return to Zero) codes are employed. FIG. 1A represents the case when the input data is "10011"; FIG. 1B represents the case where the input data is "11011"; and, FIG. 1C represents the case where the input data is "11111". It will be seen from FIGS. 1A and lB that the distortion of the optical signal wave when "1" succeeds "0" shows a relaxation oscillation pattern effect, which is considered to be due to a decrease of residual carriers in the semiconductor laser during the "0" periods. This relaxation oscillation is particularly noticeable after a long "0" period.
The optical signal is converted at a receiving side by a photo diode to an electrical signal, to identify the data "1" or "0" by a level discrimination, and thus reproduce the original data.
When a semiconductor laser is modified by high speed data of more than several Gb/s, the above mentioned pattern effect becomes notable, and if the pattern effect becomes severe, spectrum fluctuations are generated in a range whereat the relaxation oscillation is large, causing a deterioration of the received optical signal wave due to a chromatic dispersion in the optical fiber, thus increasing the discrimination errors in the received signals. The pattern effect is caused by an influence of residual carriers in the semiconductor laser. Namely, a relaxation oscillation becomes large when the residual carriers are decreased by an prolonging of the low intensity state "0" period by a series of continuous "0"s when a laser oscillation is effected by supplying a drive current.
In FIG. 1A, for example, "1" comes after two "0"s, and in this case, the residual carriers are decreased in comparison with the case shown in FIG. 1B, where "1" comes after one "0". Therefore, in the case shown in FIG. 1A, a larger relaxation oscillation and a deterioration of the optical signal wave occur than in the case shown in FIG. 1B.
In FIG. 1C, continuous "1"s are received so that the number of residual carriers is greater than a predetermined value at the times when a drive current is supplied, resulting in a small relaxation oscillation of the optical signal wave.
The problems in the conventional control system will be described in more detail with reference to FIGS. 2A to 2C.
In FIG. 2A, a dotted curve shows the relaxation oscillation at a data "1" after a series of continuous "0"s.
Further, as shown in FIG. 2B by a dotted curve, a delayed oscillation occurs in response to a rise of the input signal to "1" after a series of continuous "0"s, due to the decreased number of residual carriers in the semiconductor laser. This makes the pulse width W of the received signal narrower, and thus a deterioration of the received signal may occur after a waveform equalization thereof.
Furthermore, as shown in FIG. 2C, although the occurrence thereof has a very low probability of 10.sup.-11, the rising waveform of the oscillating signal may be fluctuated.
Moreover, after a series of continuous "0"s, chirping becomes greater and thus a deterioration of the transmitted wave form occurs. Chirping, as is well known, is a phenomenon in which the oscillating frequency is fluctuated in accordance with the carrier density in the semiconductor laser.
In a single mode semiconductor laser, when the relaxation oscillation is large, the fluctuation of the emission spectrum due to chirping or mode partitioning is increased, which causes a deterioration of the received optical signal due to a chromatic dispersion in the optical fiber, and a deterioration of eye patterns at the discriminating timings.
It is well known that the optical power can be used effectively by increasing the extinction ratio, i.e. the ratio between the high intensity state and the low intensity state optical signal levels. To increase the extinction ratio, however, the bias current supplied to the semiconductor laser must be set to a value lower than the threshold current. During modulation with high speed data, such a low bias current causes an emission delay and a large relaxation oscillation, and thus a disadvantage arises of a difficulty in producing a stable modulation drive. Accordingly, when modulation is effected with high speed data, the bias current is made high to reduce the relaxation oscillation. But, this method brings a disadvantage of a small extinction ratio.